1. Field of the Invention
The present invention relates to a torque sensor, and more particularly to a method for installing a magnetostrictive torque sensor in a measurement target.
2. Description of the Related Art
Torque measurement is performed for various use purposes in a variety of industrial fields. For example, in an automobile field, torque measurement is being put into practical use in order to detect the torque caused to occur by a driver's operation using a steering wheel in an electromotive power steering system, or to detect the torque in the rotational direction of tires when running. Provided below is the explanation about the example where the torque which is used in an automobile field and delivered to a shaft, is detected.
As a method for detecting torque, a magnetostrictive torque sensor using a magnetostrictive material is known. The magnetostrictive material has the nature that its magnetic characteristic changes as its shape is distorted. It is fixed to a target whose torque is to be measured (a shaft in this case). If torque is applied to the shaft, the shape of the magnetostrictive material fixed to the shaft is distorted. As a result, the magnetic characteristic of the magnetostrictive material changes. Therefore, the torque applied to the shaft can be detected without contact (indirectly) by detecting the magnetic characteristic of the magnetostrictive material.
As a method for fixing a magnetostrictive material to a shaft, the method for directly attaching the magnetostrictive material to the shaft and fixing it, and the method for interposing a sleeve (a support sleeve 104) between the magnetostrictive material and the shaft and fixing the magnetostrictive material as disclosed by the patent gazette TOKKAIHEI 1-97823, are known. A drawing used in the patent gazette TOKKAIHEI 1-97823 is shown in FIG. 1.
With the former method, the magnetostrictive material is fixed to the shaft by being welded or soldered, or by using an adhesive. With the method disclosed by the patent gazette TOKKAIHEI 1-97823, a measurement sleeve 101 made of a magnetostrictive material is soldered to the support sleeve 104. The support sleeve 104 is fixed by being welded using induction heating or by being solder to a shaft 105 to be measured. Note that the patent gazette TOKKAIHEI 1-97823 is a patent application, one of whose features is that the support sleeve 104 is arranged in order to reduce residual stress caused in the method of fixing a magnetostrictive material directly to a shaft.
The configuration disclosed by the patent gazette TOKKAIHEI 1-97823 aims at solving the problems of the conventional configurations as described above. However, totally judging from the sensitivity as a magnetostrictive sensor, residual stress, the degree of convenience of installation to a shaft, a bad influence of heat on the shaft, etc., there are points that must be further improved.
According to the patent gazette TOKKAIHEI 1-97823, a measurement sleeve 101 made of a magnetostrictive material is soldered and fixed to the support sleeve 104 in the state where slits are formed in measurement areas 102 and 103. With such a process, however, the magnetostrictive material is damaged by heat which is not necessary for metal composition, and the desired magnetic characteristic may not be obtained. Furthermore, residual stress is considered to occur due to a difference between the thermal expansion coefficient of the magnetostrictive material, the support sleeve and that of the solder material, although it is not large. If the magnetostrictive material is damaged by heat, the sensitivity as a magnetostrictive sensor is degraded. Additionally, if the residual stress occurs in the magnetostrictive material, it becomes difficult to correctly detect the torque delivered to the shaft. The support sleeve 104 and a shaft 105 to be measured are fixed by being welded using induction heating or by being soldered, so that the temperatures of the support sleeve 104 and the shaft 105 to be measured become high in a wide range with these methods. As a result, the residual stress is caused to occur in the magnetostrictive material when the support sleeve 104 to which the measurement sleeve 101 is installed is fixed to the shaft 105 to be measured. Additionally, since the shaft 105 to be measured, to which the torque is delivered, is tempered, its strength may deteriorate.
For welding using the induction heating, it is desirable to reduce the contact area between two objects which are to be welded. Accordingly, in the example referred to in the patent gazette TOKKAIHEI 1-97823, it seems that the contact point or the contact line between the support sleeve 104 and the shaft 105 to be measured is formed by pressing the support sleeve 104 in the direction from the outside of the support sleeve 104 to the center of the shaft 105 to be measured, and an induced current is applied to the contact point or the contact line, when the support sleeve 104 is fixed to the shaft 105 to be measured. However, if the support sleeve 104 is fixed in such a way, distortion occurs in both the support sleeve 104 and the measurement sleeve 101 to which the support sleeve 104 is fixed. As a result, there is the possibility that a correct value cannot be obtained as a torque sensor. In the meantime, if copper or silver solder is used for the fixing operation, an eddy current occurs in the place where the solder is used, which leads to weakness of an excitation magnetic field. Accordingly, the sensitivity as a torque sensor is expected to be degraded.
In a magnetostrictive material field, an annealing process is said to be effective as the method for eliminating the residual stress of the magnetostrictive material and for improving the sensitivity of the magnetostrictive material by making crystal grains larger. The patent gazette TOKKAIHEI 1-97823 does not disclose the annealing process. However, if the annealing process is assumed to be performed for the torque sensor disclosed by the patent gazette TOKKAIHEI 1-97823, the following two methods can be considered.
(1) After the annealing process is performed in the state where the measurement sleeve 101 is fixed to the support sleeve 104, they are fixed to the shaft to be measured. PA1 (2) After the measurement sleeve 101 is fixed to the support sleeve 104 and both of them are fixed to the shaft 105 to be measured, the annealing process is performed for the whole of the sensor including the shaft 105 to be measured.
Although the residual stress of the magnetostrictive material is eliminated and its sensitivity is improved with the annealing process in the above described (1), the support sleeve 104 and the shaft 105 to be measured are fixed by being welded using induction heating or by being soldered in a later process. Therefore, the temperature of the torque sensor becomes high in a wide range, and the residual stress can possibly and again occur in the magnetostrictive material. In the meantime, since the annealing process is performed at a final stage in the above described (2), the residual stress of the magnetostrictive material is eliminated and its sensitivity is improved. However, since the annealing process is also performed for the shaft 105 to be measured, the shaft 105 to be measured is damaged by the heat of the annealing process. As is often the case, a parts maker for manufacturing a torque sensor and an assembling maker for installing the torque sensor to a shaft differ. If the method referred to in (2) is adopted in this case, the assembling maker must perform the annealing process. As a result, it is inconvenient for the assembling maker. Therefore, the assembling maker may demand delivery of a torque sensor for which the annealing process is not required after installation to a shaft.